When designing machines for high speeds, the peripheral speed of the rotor and mechanical forces associated with this speed constitute a limiting factor for the rotor diameter. For the purpose of obtaining maximum magnetomotive force for a given rotor diameter, the rotor of permanently magnetized electric machines is often designed as a body of permanent-magnetic material with a circular cross section. In this way, a favourable flux distribution in the air gap of the machine, and hence a low harmonic content in the generated voltage, are also obtained. At the ends of the magnetic body end pieces are attached, for example by glueing. The end pieces are preferably of a non-magnetic material and a stub shaft for the rotor is arranged at at least one of them. For reasons of strength the magnetic body is surrounded by a capsule tube, which is also preferably of non-magnetic material. The torque transmission between the magnetic body and the stub shaft takes place via the capsule tube, which is fixed by means of a shrinkage fit to at least parts of the magnetic body and of the end piece at which the stub shaft is arranged. The shrinkage fit is so dimensioned that, under normal operating conditions as regards temperature and speed, it is capable of transmitting any mechanical torques occurring and of limiting mechanical stresses on the magnetic body.
Rotors of the above kind are known from U.S. Pat. No. 4,741,094 and from German published patent application DE 32 24 904 A1.
In U.S. Pat. No. 4,741,094 a rotor body is described which includes a solid cylindrical diametrically magnetized magnetic body arranged between two end pieces of non-magnetic material. The rotor body is surrounded by a cylindrical capsule tube, also of non-magnetic material, arranged with press fit against the magnetic body and parts of the end pieces. The rotor body is first machined into a cylindrical body, whereupon the capsule is heated and the rotor body is cooled. In this way, a difference in diameter is obtained between the two parts, sufficiently great for the parts to be joined together, whereupon the capsule tube is applied around the rotor body. When returning to ambient temperature, a hard press fit is formed between the capsule tube and the rotor body. The heated capsule tube can, however, have a harmful effect on the magnetic properties of the rotor body, and for certain magnetic materials it is necessary to make use of other methods of mounting. The patent shows a device which, by means of hydraulic pressure, brings about the radial extension of the capsule tube which is necessary for the mounting, whereupon the rotor body is inserted into the capsule tube by means of a hydraulic pressure rod. When the pressure is relieved, the elastic extension of the capsule tube is restored, whereby the desired press fit can be achieved.
In German published patent application DE 32 24 904 A1, a rotor is described which includes a rotor body with circular cross section formed of a magnetically conducting midportion, permanent magnets located on different sides of the mid-portion as viewed from the shaft, and a non-magnetic filling material between the mid-portion and the permanent magnets to obtain the circular cross-section. In addition thereto, the rotor includes a capsule tube which surrounds the rotor body and which, by being greatly prestressed, presses the parts included in the body against each other. The shaft of the machine is formed with an end plate which is welded to the capsule tube and serves as an end piece for the rotor. The rotor body is given the shape of a frustum of a cone with a conicity of about 1.degree. and the internal envelope surface of the capsule tube is formed to constitute a limiting surface for a frustum of a cone with the same conicity.
In short rotors, by which are meant in the publication rotors with a length/diameter ratio&lt;3, the capsule tube is pressed onto the rotor body and the intended prestress is checked with the aid of the diameter extension. In long rotors, the necessary pressing force can become so great because of the friction that the pressing on can only be performed with difficulty. Further, the elastic deformation of the capsule tube in the axial direction may lead to unacceptably great differences in the prestressing arising over the length of the rotor. In this case, it is proposed that the capsule tube be heated to make it possible to join the parts together. During the stage of the manufacture when the capsule tube is to be joined to the rotor body and is applied around the magnetic body and the end piece, the following aspects must be considered.
Commonly occurring magnetic materials are anisotropic, which means that their dimensional changes caused by temperature variations lead to deformations of the shape of the magnetic body. A cooling of the magnetic body is therefore unsuitable to achieve the temporary dimensional difference between the magnetic body and the capsule tube which is necessary for joining them together into one unit which limits mechanical stresses on the magnetic body and is capable of transmitting the mechanical torque developed by the machine. On the other hand, the magnetic properties of the magnetic material are sensitive to elevated temperatures, and therefore the temperature to which the capsule tube can be heated and the time during which this can be allowed to cool after having been applied around the magnetic body are limited. Further, the high-tensile steels, which are advantageously used for manufacturing the capsule tube, generally lose their properties, at least partially, when heated above certain temperature limits given by the material. In many applications in practice, it has been found that the temperature restrictions for the magnetic material and for the material of the capsule tube, that is, the upper limit to which the capsule tube can be heated, and which is related to its radial extension, leads to differences in diameter, which are unpractically small for the mounting, between the heated capsule tube and the rotor body if, at the same time, the desired shrinkage fit is to be ensured under normal operating conditions. The problems associated with elevated temperature are avoided if hydraulic pressure is utilized to bring about the desired difference in dimensions, but, on the other hand, requires a relatively complicated device for carrying out the mounting. One expedient is to manufacture the rotor body in the form of a frustum of a cone and to give at least the internal envelope surface of the capsule tube a corresponding conical shape. At least in the case of short rotors, possibilities are then opened in practice to achieve a satisfactory method of mounting the rotor also without heating the capsule tube, and particularly in combination with heating great flexibility is attained in that, because of the conical shape of the rotor parts, the temperature increase of the capsule tube can be limited to values which are harmless to the properties of the rotor. As indicated in the above-mentioned German publication DE 32 24 904 A1, the magnetic body and the capsule tube should therefore be brought against each other with a force controlled in the axial direction in order to attain the resultant shrinkage fit aimed at. Probably in order to limit the required axial force and to be able as far as possible to avoid preheating of the capsule tube, a conicity of about 1.degree. for the rotor body is suggested in the cited publication. However, simple geometrical and electromagnetic considerations show that, if it is desired to avoid the complication involved in designing the stator of the machine with a corresponding conical internal envelope surface, with increasing conicity an increasing air gap is obtained at the minor end of the rotor with a corresponding decreasing magnetic utilization of the machine. This circumstance is strengthened by the fact that rotors of this kind, which in view of their high speeds are suitably designed with a relatively small diameter and a large axial length, and by the fact that the capsule tube for reasons of strength must have its greatest wall thickness at the major end of the rotor. The cited publication DE 32 24 904 A1 only indicates that a force is applied to either the capsule tube or the rotor body to join these parts together, but to bring about a sufficient and controllable force, some form of device for achieving a controlled force must be visualized. Obvious solutions, for example a compressed-air cylinder influencing the rotor body, are, however, less suitable in that the initial acceleration on the relatively brittle magnetic body becomes high and uncontrolled, with a risk of damage to the magnetic body during mounting, if the cylinder is not provided with controlling and checking devices for its movement. In a device of this kind, where the mounting takes place with the shaft of the rotor body in the horizontal plane, the fact that the rotor body, under the influence of the force of gravity, slides on a generatrix in the capsule tube when the parts are moved against each other must also be considered.